1. Field of the Invention
The present invention relates to the field of equipment and prothesis as a circulatory device aid for assisting or replacing the right and/or the left ventricle of the natural heart. More particularly, this invention is concerned with an electrohydraulic ventricular assist device which integrates several of the major components required for this type of device into one component herein referred to as the Unified System which is implantable into the human thorax.
2. Description of the Related Art
Cardiac transplantation using a natural heart taken from a donor and grafting it into a recipient is now a relatively routine surgical technique. Recent advances have resulted in an appreciable reduction of rejection. However, practical transplantation is limited to the availability of natural heart donors and effective immunosuppressive drugs.
Clinical experience has shown that the cardiovascular circulation of patients in severe or total heart failure can be sustained with proper right and left ventricular assist devices (RVAD and LVAD).
For these and other reasons, a number of mechanical circulatory devices have been designed to replace and/or assist the diseased natural hearts. Total artificial hearts (TAH) and ventricular assist devices (VAD) have been valuable clinical tools in recent years with their primary benefit as a bridge to transplantation following acute cardiac failure. Currently many of these devices are large requiring multiple implant sites for various components and long tubes and wires to connect these various components, thus they must be located in the abdominal cavity or outside the body.
For serious cases of heart failure, one should aim for long term support rather than support limited in weeks, days or hours. In addition, it is preferable to have an assisting device which can be used for both left and/or right ventricular support. Typically, devices for assisting the left ventricle are located in the abdominal cavity or outside the chest. Traditional artificial hearts fit less than ideally inside the chest.
Generally, the requirements for a ventricular assist device are multiple and not easy to satisfy. It is desirable that such device be implanted in the thoracic cavity. The intra-thoracic blood pumping components of the device must be similar in size and weight to the natural heart. The artificial heart life must be sufficiently long and the liability sufficiently high to avoid the risk of sudden prosthesis failure. The formation of adherent thrombus mast be prevented. Thromboemboli and extensive blood damage must also be prevented. The device must not damage adjacent tissue or traumatize adjacent organs by compression or by excessive local temperatures. The artificial hearts must also avoid skin penetration by connections to the exterior to prevent infections. As well, shortening of the length of the artificial blood vessels or cannulae employed for connecting the device to the natural heart or circulatory system is another desirable requirement for the device.
There has been a great deal of development activity in the area of artificial hearts, and especially for devices to assist the left ventricle (LVADs). Generally, a ventricular assist device comprises an elastomeric blood chamber or diaphragm capped cavity. The blood chamber is provided with an inflow and an outflow valve for connection to the circulatory system. The blood chamber volume is controlled with the elastomeric diaphragm which is actuated to oscillate between a systolic and a diastolic position.
To transform electrical power generated outside the body into the rhythmic movement of the diaphragm while obtaining suitable values for the above parameters and thermodynamics, various types of energy convertors have been tested.
The displacement of the diaphragm is obtained by various methods. Among these, electrohydraulic operation of the diaphragm proves to be a dependable and reliable method. Electrohydraulic ventricular assist devices are provided with a pump which displaces a hydraulic fluid (oil) between a fluid reservoir and a fluid chamber, which is adjacent with the blood chamber so as to share the elastomeric diaphragm. The rhythmic fill and drain of the oil, in and out of the fluid chamber, displaces the diaphragm which in turn moves the blood in and out the blood chamber.
Canadian Patent No. 1,188,852 (Robinson) discloses a hydraulically actuated cardiac prosthesis with a hydraulic fluid reservoir, a hydraulic fluid pumping means and a blood pumping chamber with a flexible diaphragm. U.S. Pat. No. 4,222,127 (Donachy et al.) discloses the Pierce-Donachy artificial heart including a blood pump, flexible diaphragm and an inflow and an outflow valves which can be used paracorporeally or intra-thoracically. U.S. Pat Nos. 4,5881404 Lapeyre) and 5,089,018 (Lapeyre et al.) disclose a biventricular cardiac prothesis. The device is a sealed case with a dual membrane system for pumping the systemic and pulmonary circulations.
However, a major limitation of these past devices is their physical shape, size and complexity which increase the surgical difficulty of implanting the device in either the thorax or abdomen of a patient. Power and information transfer requirements of the past devices have also required percutaneous access to the implantation site with its associated risk of infection. Those limitations have also had an effect on the length of rime that these devices can be implanted.
Recently, devices for establishing communication of electrical signals between the implanted device and an external power source and electronics controller have been developed as disclosed in Canadian Patent application No. 2,007,439 {2,074,150} (Miller) and U.K Patent No. 2239802E (Miller). Such transcutaneous energy transformers employ electromagnetic induction using a pair of coupled coils, one outside and one inside the patient body. The ventricular assist device then can have an internal control mechanism for adjusting the frequency of oscillation of the diaphragm. To maintain maximum power transfer as the coils move relative to one another, a phase locked loop system in the external power converter maintains a constant phase relationship between voltage and current in the primary coil, thus minimizing voltage fluctuations. This was disclosed by Mussivand et al. (Performance evaluation of a transcutaneous energy transfer system, ASAIO Abstract 21:39,1992). These patents and this article are incorporated by reference.
The bidirectional communication of information between the implanted device and its portable external control unit is achieved by an infrared data link. Data is transmitted across the skin without perforating it using a standard synchronous data transmission protocol. This protocol awards hardware compatibility with any computer having an RS232 type interface. It was disclosed by Miller J. et al. (Performance Evaluation of a transcutaneous infrared telemetry system, ASAIO Abstr 21:39, 1992).
An ideal artificial heart device should integrate the major pumping and controlling components into a single Unified System for reducing the length of the electrical connections and of fluid conduits. There are also requirements for a material that should be accepted by the human body. Fluid dynamics of the device should not cause thrombus formations and the potential for thromboemboli.
Anatomical fit has a significant impact on the ease of surgical implantation, organ compression, patient comfort and postoperative complications. The internal body cavity dimensions have been recognized as a prime limitation in the design of implantable, mechanical circulatory devices.
To date no comprehensive fundamental system has been patented that provides for the integration of the major components into a compact, lightweight, totally implantable unit which can be implanted in the chest cavity.